This invention pertains generally to lasers, and more particularly to tunable external cavity semiconductor lasers. The invention is an improvement of the invention disclosed in U.S. Pat. No. 4,942,583, issued Jul. 17, 1990 to the present inventor and several co-inventors, and both inventions are commonly owned. U.S. Pat. No. 4,942,583 shall be referenced in the following as "'583", and is incorporated by reference herein.
In a diffraction grating tuned external-cavity laser, the external cavity forming the optical resonance structure has a diffraction grating at one end that may be rotated about an axis parallel to the grating rulings. The grating diffracts an incident beam into a multiplicity of spectral orders, each spread over a small range of angles about the rotation axis. One of these orders is retroflected back toward the light source by orienting the grating at the proper range of angles, known as the Littrow configuration. Wavelength selection is obtained by rotating the grating within the range of angles corresponding to this retroflected order so that the desired wavelength is reflected back to the light-emitting semiconductor, producing laser action at this wavelength.
This system is very sensitive to misalignment of the diffraction grating. Unintended rotations of the grating by small angles about an axis perpendicular to the optical axis and the direction of the rulings can drastically reduce the amount of reflected light that is fed back to the source. Such undesired rotations can be caused by misalignment of the grating rotation axis with the rulings, and by mechanical shock and vibration.
The foregoing alignment problem is addressed by Patent '583, which discloses an optical relay disposed in the external cavity beam between the semiconductor light source and the diffraction grating. A preferred embodiment of the '583 invention is illustrated in FIG. 1, which reproduces the basic features of FIG. 9 of patent '583. In this Figure the light from the semiconductor light source passes through a collimating lens, shown as a SEL-FOC lens in the drawing, then through a prism pair which expands the beam along a transverse axis (T) perpendicular to the grating rulings, and then through a cylindrical lens which contracts the beam along an axis (P) parallel to the rulings. The emerging beam is focused to a waist at the grating which has a very narrow width parallel to the rulings and a broad width perpendicular to these rulings. As disclosed in the patent, the retroreflected beam from the grating is relatively insensitive to undesired rotations of the grating about the T axis for a given sensitivity to rotations about the P axis.
The '583 system shown in FIG. 1, by addressing the alignment problem, is a great improvement over the prior art. However, the stability of an external cavity laser system is still problematic. Beam shaping elements such as the prism pair beam expander and the cylindrical lens beam contractor tend to increase the optical length of the external cavity. As the optical length of the cavity is increased, the wavelength interval between the longitudinal resonant modes of the cavity is decreased thereby degrading the mode stability and selectivity of the laser system.
As such, there is still an acute need for an external cavity laser system which is insensitive to grating rotations in the T axis and which has excellent mode stability and selectivity.